Ceramics and their production



Feb. 23, 1965 p w, McMlLLAN T 3,170,805

CERAMICS AND THEIR PRODUCTION Filed Feb. 20, 1961 A A A A zno 2O WEIGHT PERCENT bMLM 3 I MMJW (iii) o I and 'Jcao; not" greater, than .f

United States Patent r 3,170,805. v CERAMICS AND THEER PRGDUCTHUN Peter William McMillan and (iraham Partridge, Staiicrti, England,-assignors' to 'Elre'Engiisn Electric Eompany Lirnited,London, England, a British company r i Filcd ll eb'. 20, 1961, Ser. No. 90,414 Claims priority,-application Great Britain, Feb; 29, 1900,

1 9-iaims (tZl.,l0- -39) Thisdnvention relates to ceramic materials of the: kind formed from aheat-sensitive glass by controlled devitrification thereof. V Conventional fired ceramics sufier from the'disadvam' tage that the shrinkage ofthe material during drying and firing cannot be closely cont-rolled, The manu facture of fired ceramic articles to close dimensional tolerances has thus invol'ved machining the fired ceramic articles;

Recently, newceramic materials have been developed, formed from heat-sensitive glasses by Controlled devitrification thereof, Such rnaterialsare described in British Patent No. 752,243 (Corning) and in;U.S.' patent application Serial Number 42,808, filed July 14, 1960, now

abandoned, and continuation-impartapplication Serial Number 365,619, :fi'le'd May 7, 1964, in the name of the present applicants. V i

These new ceramic materials offer certain advantages over conventional fired ceramics, since, moulding and shaping techniques known in glass manufacture can be 0 2 applied to theglass articles'before devitrification, The

dimensions of the final ceramic articles can thereby be held within close limits, Furthermore, such'ceramic r'nateria'ls have substantially zero porosity, high mechanical strength, good resistance to thermal shock and high electrical resistance. The materials are hard' and are generallyresistant tovdef ormation up to temperatures, of aboutf950 C, 1 i U jln British Patent'No; 752,2;1-3 ,threis'destzribed a' heat-sensitiveglasS, the major constituents of'wh'ich lief inthe system Li OvAl O SiO The rangeroflcom-j position-shy weight described is as follows: 0 i i i Percent cent of the vglass. The remaining 10 percent of the glass may. be'ijmade up of various nonessential constituents which take .partini the glassy matrix of the-ceramic bodies togetherswithvresidual,SiOg and A1 0 The nature of;

thevn'on' essential constituents and the amou'nts by weight 5 which are permissible are; as'followsf .1

(i) Na o and K 0: not greater than 4% either alone or combined. I k (ii) nO,.SrO,;B aO, not greater than5% either "individually or collectively,

(iv)f l 3 ffnot usually greaterthan 5 %";b ut may be t0:l0%1"Wh-ei1 the, LiO content is near its maxirnumtxi i. a i

Patented fFeh 2 3, 1905 In the composition of'a heat-sensitive glass, it is es scribed in British" patent specification 'No. 2;243, a proportion of a metal was used for nucleation purposes, selected from the group consisting of 0.001 to\0.03 percent of gold computed as 'Au, 0.001 to 0.03 percent of silver computed as AgCl, and 0.001 to 1 percent of copper computed as Cu O.

According to one feature of the invention described in patent applications Serial Numbers 42,808 and 365,-

619, there is described a heat-sensitiveglass, the major constituents of ,Whichlie in thesystem V Li 0;-MgO Al O SiO A The range of compositionsgby weight describedis'asfollows:

. 0-36.0 sio 45.0-88.0

In addition to the above major constituents, which should total at least of the composition, various constituents of secondary importance could hep-resent as follows:

percent either alone or combined. (ii) Zinc oxide (ZnO) up to 10 weightpercent. 7 (iii) Calcium oxide (CaO) up to 5 weightjpercent. f

v 0 (iv) Bor'ic oxide (B 0 up to 10 weight percent l, According to one aspect of'the inventiondescribed in: patent application SerialNumber 42,808, filedrJuly 14, 1960, now abandoned, and continuation-in-part application Serial Number 365,619, filed May 7 19 64, nuclea-Z' tionof a heat-sensitive glass is provided by-phospl'iate'- ions.

. According to the present invention, a ceramic material; l

is formed by controlled devitrificationcfia heat-sensitive glass, the major constituents of which, liein the systemi v A glass of this composition is dis ting'uished from a glass of any 'of thecompositions. de-j scribed above, in which zinc oxide is present optionally,

as one of various non-essentialconstituents.

The proportions by Weight of these major.cons tituents-,' which make up at least 90 percent by weight ofthe' total composition, are as follows: i i

This range of compositions is shown by the enclosed area A in the accompanying drawing.

A preferred rangeof compositions is. as follows; H

" Percent This range of compositions is shown' 'r'by theienclosed" area B in the accompanying drawingl}, r

In addition tothe above majorl c onstituent s, certain Weight percent V constituents of secondary importance may also be present. The nature and proportions of these are as follows:

(iii) Either phosphates or metallic nucleating agents, for example, gold, silver or copper may be used in glass compositions inthe whole of the present range. These heat-' sensitive glasses can then be converted by suitable heattreatment into satisfactory ceramics. 'The phosphate nucleating agent is added in such quantity that the quan tity of phosphate anions introduced into the resultant glass corresponds to 0.5 to 6.0 percent by weight of phosphorus pentoxide (P The metallic nucleating agent is selected from the group consisting of 0.02 to 0.03 percent of gold computed as Au, 0.02 to 0.03 percent of silver computed as AgCl, and 0.5 to 1.0 percent of copper computed as Cu O.

A description will now be given, by way of example, of the preparation of a number of heat-sensitive glasses and the manner of their controlled devitrification into ceramic materials according to the present invention. There then follows a description of the physical properties of the resultant ceramic materials and examples of their use.

(a) Preparation of the heat-sensitive glasses.-For the preparation. of the glasses, .the following batch materials are used for the major constituents:

Lithium carbonate, Li CO Zinc oxide, ZnO Ground quartz, SiO

and for the secondary constituents:

ing. As a further constituent of the batch, a suitable. quantity of a nucleating agent is added. For glasses nucleated with phosphate anions, the phosphate of a metal is added, so that the quantity of phosphate anions introduced into the resultant glass corresponds with 0.5 to 6.0 percent by weight of phosphorus pentoxide. The

phosphate may be added as the orthophosphate of a metal occurring in the glass composition, phosphates being:

Lithium orthophosphate, Li PO Zinc orthophosphate, Zn (PO the preferred The orthophosphates of sodium, potassium aluminium and magnesium maybe used 3l'ld 8.lSO the meta-phosphates and pyro-ph'osphates of any of the above metals.

Examples of compositions nucleated with phosphate anions and the method of adding the phosphate are as follows: i I

(i) Composition No. 1:

phosphate. I 1 (ii) Composition No: 2: phosphate added as lithium orthopho'sphate.

phosphate added as zinc ortho- The numbers of the compositions refer to the numbers given in the appended Table I.

For compositions containing gold, silver or copper which are introduced as 0.02 to 0.03 percent of gold computed as An, 0.02 to 0.03 percent of silver computed as AgCl, and 0.5 to 1.0 percent of copper computed as Cu O the constituents are added to the batch as follows:

Gold chloride solution Silver nitrate solution Cuprous oxide Composition No. 9, in the appended table, is an example of a composition using gold for nucleation purposes.

The batch compositions when thoroughly mixed, may be melted in crucibles of the fireclay, sillimanite, or high zircon type using either electric melting furnaces or gasfired melting furnaces.

Where phosphates are used in the glass compositions,

the melting furnace atmosphere is not important, as the furnace atmosphere has no effect on the phosphate ions. With the metallic nucleating agents the furnace atmosphere must be controlled as follows:

Gold-oxidising or neutral conditions. Silver-oxidising or neutral conditions. Copper-reducing conditions.

The melting temperatures used range from 1200 C. to 1400 C., the melting temperature being so arranged as to enable the glass to be obtained seed and batch free 'and'with a minimum of crucible attack. For example,

Composition No. 3 may be melted at 1200 C. whilst Composition No. 9 requiresv a melting temperature of 1350 C. to 1400 C.

After refining the glasses may be worked by the normal methods employed in glass working, such as casting, drawing and pressing. The samples obtained may then be annealed at a suitable temperature depending on the glass composition. For example, Composition No. 3 would be annealed at 450 C. whilst Composition No. 6 would beannealed at 550 C. Alternatively, the samples may be immediately subjected to the heat-treatment processes as described below.

(b) Conversion of the glasses into ceramics-The glass samples are subjected to heat-treatment processes as described below in order to convert the glasses to ceramic materials.

(i) The articles prepared as described above are heated in a furnace at a rate of 4-5" C. per minute to the Mg point of the glass as determined dilatometrically, 0r, if the samples are to be heat-treated'immediately after preparation, they are transferred direct to a furnace maintained at the Mg point of the glass. The Mg point is related to the point of the maximum expansion of the glass, and has been defined in McMillan et aLpending application Serial No. 90,210. This temperature is maintained for a period of at least one hour and this treatment serves to nucleate the glass and also to initiate the crystallisation processes so that, during subsequent heating, the article is sufficiently refractory to maintain its shape. I

(ii) The heating is continued, raising the temperature of the article at 4-5 C. per minute to the final crystallisation temperature which is maintained for at least one On completion of the heat-treatment, the ceramic sampics are allowed, to cool at the normal cooling'rate of the furnace. However, experience has shown that the; ceramics will withstand quite rapid rates of cooling. .For

' tions.

tions, for which the above ceramics areconsidered to be (b)- Vacuumse'al components, for which ceramicsexample, cooling rates as highas 600" C. per hour do not cause fractures.

The ceramic materials formed by controlled devitrifica- 'tion of glass compositions derived from the system are micro;crystalline and may be obtained without deformation occurring during the heat-treatment processes. ceramics vary with thecompositions of the ceramics. The types of crystals which have 'been shown to be present by means of X-ray diffraction analysis are:

Lithium disilicate, Li O.2SiO Zinc orthosilicate, 2ZnO.SiO Alpha cristobalite, SiO

The ceramics prepared contain one 'or more of the crystal types listed, depending on the chemical composition, and in addition a glassy phase is present in each case.

Examination of the ceramics by means of the electron microscope has revealed that the crystals range in size from 0.1 to 6.0 microns. They are irregular in shape and are closely interlocked, which makes for a strong, dense material.

The ceramics formed using phosphate ions for nucleation purposes are all white in colour, those using gold are red in colour, those using silver are grey in colour and those using copper are pink in colour.

The types of crystals formed within the particular thermal expansion coeflicients are required to match those of known metals or alloys.

TABLE I Weight percentage compositions of selected glass-ceramics Composition No; Constituants .7 9.6 15.1 i 9.3 11.4 .5- 30.2 30.2 48.1 41.3 50.5 14.9 .s 48.2 44.6 44.4 40.6 37.2 62.3 P2o5 3.0 3.0 30 3.0 3.0 "5:0"5'0' Au .027

To further illustrate the present invention the following example is presented as applicable to Composition No. 1 in the above Table I.

The batch materials to give this glass arethoroughly 1 mixed and are then melted at a temperature of 1300 C.

in a high zircon type refractory. 'The glass is refined until seed and batch free, is then Worked by normal glass working procedures to form desired shapes, and is then annealed at 480 C.-until stress-free.

The ceramics formed by devitrification of glasses of the Li O-ZnOSiO system,- as the following properties;

The linear thermal expansion coeflicients of the ce-f ramics cover a'wide range, since ceramics ranging in thermal expansion coefficient from 42.6 -10' to 1740x10 v (20-'500 C.) have been prepared. a

.The refractoriness of the ceramics varies.' Some ceramic s resist deformation at temperatures up to 800 C. (Composition No. 9), whilst others resist deformation at temperatures in excess of 1000" C. (Composition No. 7).

The mechanical strength of ceramics derived from glasses of the Li OZnO'SiO systemhave been shown to be high and values up to 40,000 lb./in. have beenohtained on rod samples 5 mm. diameter, using a 3-point loading method with a' loading length of 1.50 inches, and values up to 90,000 lb./in. on rod samples 4 mm. di-

' ameter using a 3-point loading method with a loading length of 1.50 ems.

The. electrical properties of ceramics derived from described above, have Article formed above are heat-treated at a temperature of 500 C. for one hour to nucleate the glass and initiate the crystallisation processes and subsequently; at a temperature of 850 C. for 1 hour to complete the crystallisation processes, the heating ratesbetween hold- U ing stages being 5? C. per minute and the cooling rate to roomtemperature being less than l0 C per minute.

-We claim:

1. A ceramic material formedby controlled devitriiication of a heat-sensitive glass consisting of major con- 1 stituents which make up at least 90 percent 'by'weight of the total composition and lie in the system defined by Li O2.0 to 27.0 percent, ZnO-10.0 to 59.0 percent,

and SiO -.-34.0to 81.0 percent; minor constituents which:

consist of an oxide of atleast one ofthe metals selected from the group consisting of sodium potassium,

aluminium,'; magnesium, calcium, barium, lead and boron; and a nucleating agent'selected from the group consists ing of gold, silver, copper and phosphate ion.

glasses of the LiOZnO SiO system have been shown to be-good, values of the dielectric breakdown strength (on D.I.N. type samples, testing thickness 1 mm.) have been obtained within the range 34-50 kv/mm- Values of'loss angle in the range 13 10* to 60 10- 'at a frequency of 1 mc./s. and values of permittivity in the suitable. m I 1 (a) High-tension insulatingfciomponents and insulating 2. A ceramic material as claimed in claim 1, in which the said major constituents lie" in the system defined by; Li Of7.O to 25.0 percent, ZnO-'10.0.to 30.0 percent and SiO 50.0 to 79.0 percent 3; A ceramic material as claimed in claim 1, in which ,.the said minor constituents include at least one 'of-the into the glass corresponds to 0.5 to 6.0 percent by weight L of phosphorus pentoxi'de.

substances selected from the group; consisting of alkali metal oxides'Na O and K 0 up .to 5 weight percent in total, aluminium oxide A1 0 up to 10 weight percent, magnesium oxide Mg'O up to 10 weight percent, calciumoxide Cat) and barium oxide BaO up to 5 weight percent in total, boric oxide B 0 up to '10 weight percent,

and'lead oxide PbO up to-5 weight percent. 4 4. A ceramic material as claimed in'claim 1, in whic the nucleating agentis phosphate ionpresent insuch amount that the quantity'of phosphate, ions introduced .5. A ceramic material as claimed in claim 1, in which thenucleating agent is 0.02 to 0.03 percent of gold com-4 puted as Au. 1

6. A ceramic material as claimed-in claim 1, in which the. nucleating agent is 0.02 to 0.03 percent of silver cornputed as AgCl.

componentsin generalgwhere' an easily shaped ceramic is required,having highmechanical strength combined with satisfactoryelectricalproperties.

7 its.-

7. A ceramic material as claimed in claim "1, in which i the nucleating agent is 0.5 to 1.0 percent of copper com-' puted as C11 0. I i

r 8. ,A process for manufacturing a .ceramicmaterial as claimed in claim 1, consisting essentially of the steps of heating the batch composition to a temperature in the range 1200 C. to 1400 0., allowing the resultant glass to cool, and subsequently heat-treating the glass to bring about its devitrification by maintaining its temperature at the Mg point for a period sufficient to nucleate the glass and to initiate crystallization thereof and subsequently maintaining the temperature at a value between 800 and about 1000 C. until crystallization is completed.

' 9. A process as claimed in claim 8, Consisting essentially of the step of forminga shaped article by a glassworking operation carried out prior to the heat treatment for bringing about devitrification.

til

0 References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Hinz: Chemical Abstracts, Item 12615c, July 10, 1959,

Vitrokeram.

Eitel et al.: Glastechnische Tabellen, published, 1932,

by Springer, Berlin, page 41. 

1. A CERAMIC MATERIAL FORMED BY CONTROLLED DEVITRIFICATION OF A HEAT-SENSITIVE GLASS CONSISTING OF MAJOR CONSTITUENTS WHICH MAKE UP AT LEAST 90 PERCENT BY WEIGHT OF THE TOTAL COMPOSITION AND LIE IN THE SYSTEM DEFINED BY LI2O-2.0 TO 27.0 PERCENT, ZNO-10.0 TO 59/9 PERCENT, AND SIO2-34.0 TO 81.0 PERCENT; MINOR CONSTITUENTS WHICH CONSIST OF AN OXIDE OF AT LEAST ONE OF THE METALS SELECTED FROM THE GROUP CONSISTING OF SODIUM, POTASSIUM, ALUMINIUM, MAGNESIUM, CALCIUM, BARIUM, LEAD AND BORON; AND A NUCLEATING AGENT SELECTED FROM THE GROUP CONSISTING OF GOLD, SILVER, COPPER AND PHOSPHATE ION. 